In the field of aeroelasticity, the phenomenon known as flutter generally refers to a condition produced by the coalescing and proper phasing of two or more structural vibration modes of a structure, such as an aircraft wing, fuselage, empennage, or other structural component. During flight, the aerodynamic forces over such structures may cause an unstable oscillatory aeroelastic deformation of the structure referred to as flutter. Flutter of an aeroelastic structure may depend on numerous factors, including the mass, stiffness, and shape of the structure, and the particular operating conditions of the structure, including the velocity and density of the airstream.
Flutter of an aircraft wing may involve both bending and torsional types of motion. In some cases, the aeroelastic deformations may be relatively mild and stable within the normal operating envelope of the aircraft. In the case of flutter, however, the aeroelastic deformations are driven into an unstable mode in which the torsional motion extracts energy from the airstream and drives the bending mode to increasingly higher amplitudes, causing oscillations of increasing amplitude that may eventually result in catastrophic failure of the structure.
The avoidance of the unstable condition of flutter and the determination of the maximum allowable flight speed before flutter is encountered are critical priorities for designers of aeroelastic structures and aerospace vehicles. Exhaustive flight and wind tunnel tests are usually conducted to record and observe the flutter characteristics of the various aeroelastic structures of an aircraft over the entire flight envelope of the vehicle , and to predict a safe operating speed envelope. This is typically accomplished by determining the frequency and damping of each important aeroelastic vibration mode, and tracking the changes in these parameters for a variety of Mach number and dynamic pressure flight conditions. For military aircraft combat and surveillance missions, the attachment of multiple external stores to the wing or fuselage further complicates the analysis and increases the extent and complexity of the testing required. Differences in store number, type, and mounting location give rise to complex multi-variable oscillation coupling patterns, and can give an aircraft as many different flutter speeds as there are store configurations.
The reduction of flutter test time history data is difficult for three principal reasons. First, the test data from flutter sensors can contain significant noise which may hamper the analysis. Second, the test data may contain multiple modes, each with different frequency and damping characteristics. And third, because typical flutter testing involves the acquisition of data from many sensors distributed over an aeroelastic structure, the data from different sensors may yield different results. For example, each sensor or transducer may respond in one or more of the aircraft's structural modes (resonance frequencies). For a single transducer, the response is often dominated by one mode. However, the response of other aircraft modes can make it difficult to extract the characteristics of the primary mode. The presence of noise in the data only exacerbates the problem. In the past, several transducers would be processed individually, and results might necessarily be selected from a single transducer. Alternately, averaging among the available data may be performed. These analytical methods tend to rely on the subjective determinations of a test engineer. Although desirable results have been achieved using such analysis methods, the complexity of aeroelastic flutter and continuing advances in aerospace vehicle requirements are placing increasing demands on those who design and analyze flutter test data of aeroelastic structures. Therefore, there is an unmet need in the art for improved methods of analyzing flutter test data that might better characterize the data from a plurality of transducers with reduced subjectivity.